The invention relates to a radiographic installation comprising a film support, transportable from a readiness position into at least one exposure position and back again, with displaceable clamping jaws for the support-mounting of x-ray film cassettes of varying format.
Through the German AS No. 20 44 848 an x-ray spot film device or x-ray cassette changer is known in which an x-ray film cassette is transportable by motor means from a readiness position into an exposure position. In the case of this x-ray spot film device or x-ray cassette changer, voltage dividers are adjusted by the clamping jaws which can be brought to rest against the cassette edges. The adjusted resistance values, together with a constant value for the path specification, are connected with a motor-driven follow-up control for the control of the movement of the cassette carriage from the readiness position into the exposure position. These resistance values can also be connected to an additional follow-up control for the purpose of preadjustment of the collimator. It is a peculiar feature of this construction that the voltage dividers, which convert the sensed cassette dimensions into electrical values, must have an electric connection with the radiographic installation and, in the case of a corresponding control of the collimator, must also have an electric connection with the remaining x-ray examination apparatus. This has as a consequence the fact that the voltage dividers coupled to the clamping jaws must either be rigidly wired with the remaining x-ray examination apparatus, or that they are to be connected to the latter by means of a plug-in connection. Both prevent or obstruct the removal of the film support.
The German patent No. 24 15 410 which corresponds to U.S. Pat. No. 3,986,034 disclose a cassette plate insertable beneath an examination table in which the clamping jaws, during abutment on the cassette edges, adjust a permanent magnet displaceably mounted transversely to the insert direction, via a cable pull in the cassette plate. This permanent magnet activates, depending upon the position of the inserted cassette plate, one of several reed contacts installed in the longitudinally running carriage of the examination table. This reed contact then connects a corresponding resistance in a follow-up control for the collimator and thus adjusts the latter to the sensed cassette dimensions. It is a peculiar feature of this construction that although the cassette plate manages without electric connections and therefore can be readily removed from the examination table, it can, in exchange, be employed only in the case of specific, discrete, matched cassette dimensions.
In order to be able to adjust the collimator also to x-ray film cassettes of a random format, it has become known from the German OS No. 27 44 139 to mount sensors on the frame-shaped longitudinally running carriage remaining in the examination table which, during insertion of an x-ray film cassette clamped on a cassette plate, abut externally on the clamping jaws of the cassette plate and, via a gear, adjust one separate potentiometer each for the width and for the height of the x-ray film cassette. Such sensors must be relatively narrow and long and therefore have the property of readily bending. Thus, either the insert opening of the cassette plate is blocked, or, as a consequence of sliding past the clamping jaws of the cassette plate, erroneous exposures result. Moreover, a considerable fine-mechanical outlay for the transmission of the movement of the sensors to corresponding potentiometers is necessary. Even minor angle changes can lead to clearly measurable resistance changes.